1. Field of the Invention
This application relates to a unique structure for a filter as typically used in microfluidic devices and a method of manufacturing such a filter and, more particularly a unique structure for a filter having particular use in an ink jet printer system, i.e. having reduced fluidic pressure drop across the filter with fluid flow.
2. Description of the Prior Art
There is a trade-off in filter design between flow resistance and filter effectiveness especially for small particulate size. For a given minimum distance between circular filter pores, the percent open area of the filter is decreased for small diameter pores. In thermal ink jet systems, for example, the implication for small enough pore size is that the printing frequency might be limited by the flow through the filter. For various drop sizes and printing frequencies simple patterns of circular pores are adequate. However, there is a general interest in going to smaller drop sizes e.g. (requiring a finer filter) and higher frequencies in the order of 15 khz and higher.
In new areas of microfluidics, microfluidic carrying devices and their components are small, typically in the range of 500 microns down to as small as 1 micron, and possibly even smaller. Such microfluidic devices pose difficulties with regards to preventing fluid path blockage within the microscopic componentry, and especially when the particular microscopic componentry is connected to macroscopic sources of fluid. Yet such microfluidic devices are important in a wide range of applications that include drug delivery, analytical chemistry, microchemical reactors and synthesis, genetic engineering, and printing technologies including a wide range of ink jet technologies, such as thermal ink jet printing.
A typical thermally actuated drop-on-demand ink jet printing system, for example, uses thermal energy pulses to produce vapor bubbles in an ink-filled channel that expels droplets from the channel orifices of the printing system""s printhead. Such printheads have one or more ink-filled channels communicating at one end with a relatively small ink supply chamber (or reservoir) and having an orifice at the opposite end, also referred to as the nozzle. A thermal energy generator, usually a resistor, is located within the channels near the nozzle at a predetermined distance upstream therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. A meniscus is formed at each nozzle under a slight negative pressure to prevent ink from weeping therefrom.
Some of these thermal ink jet printheads are formed by mating two silicon substrates. One substrate contains an array of heater elements and associated electronics (and is thus referred to as a heater plate), while the second substrate is a fluid directing portion containing a plurality of nozzle-defining channels and an ink inlet for providing ink from a source to the channels. This substrate is referred to as a channel plate which is typically fabricated by orientation dependent etching methods.
The dimensions of ink inlets to the die modules, or substrates, are much larger than the ink channels; hence, it is desirable to provide a filtering mechanism for filtering the ink at some point along the ink flow path from the ink manifold or manifold source to the ink channel to prevent blockage of the channels by various particles typically carried in the ink. Even though some particles of a certain size do not completely block the channels, they can adversely affect directionality of a droplet expelled from these printheads. Any filtering technique used should also minimize air entrapment in the ink flow path.
Various techniques for forming filters are disclosed in the prior art U.S. Pat. Nos. 5,124,717, 5,141,596, 5,154,815 and 5,204,690 disclose fabrication techniques for forming filters integral to a printhead using patterned etch resistant masks. This technique has the disadvantage of flow restriction due to the proximity to single channels and poor yields due to defects near single channels. These patents are intended to be incorporated by reference herein in their entirety.
U.S. Pat. No. 4,864,329 to Kneezel et al. discloses a thermal ink jet printhead having a flat filter placed over the inlet thereof by a fabrication process which laminates a wafer size filter to the aligned and bonded wafers containing a plurality of printheads. The individual printheads are obtained by a sectioning operation, which cuts through the two or more bonded wafers and the filter. The filter may be a woven mesh screen or preferably a nickel electroformed screen with predetermined pore size. Since the filter covers one entire side of the printhead, a relatively large contact area prevents delimitation and enables convenient leak-free sealing. Electroformed screen filters having pore size which is small enough to filter out particles of interest result in filters which are very thin and subject to breakage during handling or wash steps. Also, the preferred nickel embodiment for a filter is not compatible with certain inks resulting in filter corrosion. Finally, the choice of materials is limited when using this technique. Woven mesh screens are difficult to seal reliably against both the silicon ink inlet and the corresponding opening in the ink manifold. Further, plating with metals such as gold to protect against corrosion is costly. This patent is intended to be incorporated by reference herein in its entirety.
In all cases, conventional filters ordinarily suffer from blockage by particles larger than the pore size, and by air bubbles. Conventional filters used for thermal ink jet printheads help keep the jetting nozzles and channels free of clogs caused by dirt and air bubbles carried into the printhead from upstream sources such as from the ink supply cartridge. One common failing of all filters is that dirt can accumulate on the filter surface causing restricted fluid flow. Another kind of blockage is when an air bubble rests on the filter surface thereby covering a large group of fluid flow holes preventing any fluid from passing through that region of the filter.
In laser ablated filters, which have been described in commonly assigned U.S. Pat. No. 6,139,674, to Markham et al for a Method Of Making An Ink Jet Printhead Filter By Laser Ablation and co-pending U.S. patent application Ser. No. 09/431,056, filed Nov. 1, 1999, circular holes are laser ablated in a plastic film, which may then be bonded over the ink inlets of many die at once in a thermal ink jet wafer. However, even when the holes are packed as tightly as possible, the open area for typical filter dimensions may be on the order of 40%. This patent and patent application are incorporated by reference herein in their entirety.
FIGS. 3A and 3B illustrates two arrays of circular holes 10 as found in known filter configurations. In FIG. 3A an array of laser ablated holes 10 is shown on a square grid. Each hole 10 has a diameter d, and the spacing between holes 10 is s in both the X and Y directions. FIG. 3B illustrates a hexagonal close packed array of laser ablated holes 10. Each hole has a diameter d. The minimum spacing between adjacent holes is s. Thus in both cases the hole diameter is d and the spacing between adjacent holes is s. In FIG. 3A the holes are on a square grid, while in FIG. 3B the holes are more tightly packed on a hexagonal close packed grid. For example, for a 600 spi thermal ink jet color printhead which shoots a drop size of 10 pl, a pore diameter of d=10 microns has been found to be capable of blocking particulates which could potentially clog jets, while also not restricting the printing frequency below 12 kHz. (Actually there is a taper of the hole size going through the typically 25 micron thick Upilex plastic film, but 10 microns is the nominal diameter.) A typical spacing between pores is s=5 microns. A simplified dimensional analysis of filter flow capabilities indicates that the flow resistance of a close-packed filter depends on the pore radius. For a simple filter, the flow resistance goes as 1/(nr4) where n is the number of pores and r is the pore radius. The number of pores goes approximately as rxe2x88x922 so that the flow resistance goes as rxe2x88x922, assuming efficient packing. Since for round hole filters one is limited to a diameter somewhat smaller than the minimum opening in an ink jet channel, there is a significant limitation in the sustainable flow rate through a filter given a certain area available for the filter.
In an ink jet system environment, one of the basic objectives of the embodiments of the present invention is to provide a filter which will prevent particulate matter of a size sufficient to block channels from entering the printhead channels, improve ink droplet directionality in an ink jet printhead (i.e. improve functionality of the ink jet system) and minimize air entrapment along the ink flow path. In order to decrease the flow resistance due to the filter, it is also an objective of the embodiments of the present invention to make pores that are narrow enough in one dimension to exclude particles large enough to block channels, but elongated in the other dimension, In order to improve the filtration effectiveness for anisotropic particles, the following pore geometries are incorporated within the features of the present invention: a) slots with constrictions along the length of the slot, or b) slots which have curvature along their length (sinusoidal or saw-toothed, for example). The optimal length of the slots will be determined not only by open area and filtration effectiveness, but also by mechanical strength of the filter. In accordance with features of the present invention, there will need to be occasional bridges between slots.
To achieve the advantages described herein in accordance with the purpose of the invention, the inventive features as embodied by the present invention include a filter structure having a plurality of pores through the structure, each of the pores having a cross-section with a length (L) and a width (W) wherein the dimension of L is greater than the dimension of W. In accordance with the features of the present invention the pore structure is formed by laser ablation.
Another embodiment of the present invention is directed to an improved jet printhead having an ink inlet in one of its surfaces, a plurality of nozzles, individual channels connecting the nozzles to an internal ink supplying manifold, the manifold being supplied ink through the ink inlet, and selectively addressable heating elements for expelling ink droplets, the improved ink jet printhead comprising a substantially flat filter having predetermined dimensions and being bonded to the printhead containing the ink inlet, the filter having a plurality of pores, therethrough each of the pores having a length=L and a width=W, wherein the dimension of L is greater than the dimension of W.
Still another embodiment of the present invention is directed to a method for fabricating a filter element comprising the steps of: positioning a thin polymer film in the output radiation path of an ablating laser, and controlling the laser output so that a plurality of pores are formed in portions of the polymer film; each of the pores having a cross section with a length (L) and a width (W), wherein the dimension of L is greater than the dimension of W.